organism/neuron/appunti/2026-02-05-Pr-Presynapse-behaviours.md

Presynaptic Release Model: Sequence of Events & Multi-Timescale Modulations

CORE RELEASE SEQUENCE (Milliseconds)

Event 1: Action Potential Arrival

Time: t = 0 ms
Trigger: Depolarization from axon hillock
Mechanism: Na⁺/K⁺ voltage-gated channel cascade
State: Terminal depolarizes from -70 mV to +30 mV
Duration: ~1 ms

Event 2: VGCC Opening

Time: t = 0.2-0.5 ms after AP arrival
Trigger: Membrane depolarization > -40 mV
Types: CaV2.1 (P/Q-type), CaV2.2 (N-type) at active zone
Mechanism: Voltage sensor movement → pore opening
Open probability: ~0.3-0.8 during AP peak
Open duration: ~0.5-2 ms

Event 3: Ca²⁺ Influx & Microdomain Formation

Time: t = 0.3-1 ms after AP arrival
Ca²⁺ source: Extracellular (1.2 mM) → intracellular (50 nM baseline)
Influx rate: ~3000-10000 Ca²⁺ ions per VGCC per ms
Microdomain formation:
  - Within 20 nm of VGCC: 10-100 µM
  - At vesicle release site: 10-25 µM threshold for release
  - Rise time: <100 µs
  - Diffusion-limited spread: ~100-200 nm radius

Event 4: Vesicle Release Decision

Time: t = 0.4-1.5 ms after AP arrival
Decision conditions (AND logic):
  1. [Ca²⁺]microdomain > 10-25 µM
  2. Vesicle in RRP (docked & primed)
  3. SNARE complex in primed state
  4. Synaptotagmin Ca²⁺ binding sites occupied
Mechanism:
  - Ca²⁺ binds synaptotagmin (4+ cooperative sites)
  - Synaptotagmin displaces complexin clamp
  - SNARE complex completes zippering
  - Vesicle membrane fuses with presynaptic membrane
Release latency: 0.1-1.0 ms after Ca²⁺ threshold reached
Release synchrony: Multiple vesicles can release simultaneously

Event 5: Ca²⁺ Clearance

Time: t = 1-50 ms after influx
Clearance mechanisms (in order of speed):
  1. Fast buffers (calbindin, parvalbumin): <1 ms
  2. Plasma membrane Ca²⁺ ATPase (PMCA): 10-100 ms
  3. Na⁺/Ca²⁺ exchanger (NCX): 10-100 ms
  4. Mitochondrial uptake: 10-1000 ms
  5. Endoplasmic reticulum uptake: 100-1000 ms
Residual Ca²⁺: 0.1-0.5 µM persists for 10-1000 ms

Event 6: Vesicle Recycling

Time: t = 10 ms - 10 s (depending on activity)
Sequential steps:
  1. Endocytosis (clathrin-mediated, kiss-and-run, bulk)
  2. Vesicle re-acidification (v-ATPase)
  3. Neurotransmitter reloading (vesicular transporters)
  4. Priming (SNARE assembly, docking)
  5. Return to RRP
Recycling rate: Limited by ATP availability

MODULATION MATRIX ACROSS TIMESCALES

MODULATION LEVEL 1: MILLISECONDS (Immediate Signal Processing)

Event	Modulator	Effect	Mechanism	Timescale
VGCC Opening	Voltage waveform	AP width/duration affects open time	K⁺ channel activation shapes repolarization	During AP (1 ms)
VGCC Opening	Ca²⁺-dependent inactivation (CDI)	Auto-inhibition	Ca²⁺ binding to calmodulin on VGCC	5-50 ms
Ca²⁺ Microdomain	Endogenous buffers	Shapes [Ca²⁺] spatial/temporal profile	Calbindin, parvalbumin binding	<1 ms
Release Decision	Synaptotagmin isoforms	Different Ca²⁺ sensitivity	Cooperative binding sites vary	During Ca²⁺ binding
Release Decision	SNARE complex state	Priming level determines speed	Munc13 activity, NSF disassembly	<1 ms
Ca²⁺ Clearance	Fast buffers	Immediate Ca²⁺ binding	High-affinity Ca²⁺ binding proteins	<1 ms

MODULATION LEVEL 2: TENS-HUNDREDS OF MILLISECONDS (Short-term Plasticity)

Event	Modulator	Effect	Mechanism	Timescale
VGCC Opening	Residual Ca²⁺	Ca²⁺-dependent facilitation	Ca²⁺/CaM binding to VGCC β-subunit	10-1000 ms
VGCC Opening	eCB (CB1 activation)	Direct inhibition	Gβγ subunit blocks pore	100-2000 ms
Ca²⁺ Microdomain	Mitochondrial uptake	Buffer capacity changes	Uniporter activity with Ca²⁺ buildup	10-1000 ms
RRP Replenishment	Residual Ca²⁺	Enhanced mobilization	Calcineurin activation → synapsin dephosphorylation	10-1000 ms
RRP Replenishment	ATP availability	Limits priming rate	NSF, v-ATPase, priming ATPases	Continuous
Release Probability	mGluR autoreceptors	Activity-dependent inhibition	Gi/o → ↓cAMP → ↓PKA → ↓VGCC	50-500 ms
Ca²⁺ Clearance	PMCA/NCX pumps	Activity-dependent saturation	Limited pump capacity during high frequency	10-1000 ms
Vesicle Recycling	Activity level	Different endocytosis modes	High activity → bulk endocytosis	100 ms - 10 s

MODULATION LEVEL 3: SECONDS-MINUTES (Metabolic & Signaling)

Event	Modulator	Effect	Mechanism	Timescale
VGCC Density	Surface expression	More/fewer channels	Phosphorylation-dependent trafficking	30 s - 5 min
VGCC Function	Kinase phosphorylation	Changed open probability	PKA, PKC, CaMKII phosphorylation	1-10 min
Ca²⁺ Dynamics	NO (cGMP pathway)	Altered pump activity	PKG phosphorylation of PMCA	1-10 min
Vesicle Pools	Mobilization rate	RP→RRP speed changes	Synapsin phosphorylation state	1-10 min
ATP Production	Lactate shuttle	Enhanced ATP supply	Astrocyte-neuron metabolic coupling	1-10 min
Release Machinery	BDNF (acute)	Enhanced priming & docking	TrkB → PLCγ → PKC activation	1-10 min
Autoreceptor	mGluR desensitization	Reduced autoinhibition	β-arrestin recruitment, internalization	1-10 min
CB1 Receptor	eCB-induced desensitization	Reduced inhibition	GRK phosphorylation, internalization	1-10 min
Ca²⁺ Clearance	Mitochondrial adaptation	Increased capacity	Matrix Ca²⁺ activates dehydrogenases	1-10 min

MODULATION LEVEL 4: MINUTES-HOURS (Structural & Transcriptional)

Event	Modulator	Effect	Mechanism	Timescale
VGCC Expression	BDNF (chronic)	Increased channel synthesis	TrkB → MAPK → CREB → gene expression	30 min - 6 hr
VGCC Expression	Activity-dependent	Homeostatic scaling	Ca²⁺ → NFAT/CREB → VGCC genes	1-24 hr
Active Zone Size	BDNF, activity	More release sites	Structural proteins (RIM, Bassoon) synthesis	1-12 hr
Vesicle Pool Size	BDNF, activity	Larger RP/RRP	Synaptic vesicle protein synthesis	1-12 hr
SNARE Machinery	Local translation	More fusion complexes	mTOR-dependent protein synthesis	30 min - 3 hr
Mitochondrial Density	BDNF, activity	Increased ATP capacity	PGC-1α activation → biogenesis	3-24 hr
Metabolic Pathways	Activity patterns	Enhanced glycolysis/oxidation	HIF-1α, AMPK pathway activation	1-6 hr
Receptor Expression	Chronic modulation	mGluR, CB1 level changes	Transcriptional regulation	3-12 hr
Buffer Expression	Ca²⁺ history	Calbindin/parvalbumin levels	Ca²⁺-dependent gene regulation	3-12 hr

MODULATION LEVEL 5: HOURS-DAYS (Metaplasticity & Structural Remodeling)

Event	Modulator	Effect	Mechanism	Timescale
Synapse Size	Chronic activity	Growth/shrinkage	Cytoskeletal reorganization	6-48 hr
VGCC Distribution	Activity patterns	Clustering changes	Scaffold protein (Homer, Shank) expression	12-72 hr
Release Site Organization	BDNF, activity	Active zone architecture	Piccolo, Bassoon, RIM expression	12-72 hr
Vesicle Recycling	Chronic patterns	Efficiency changes	Endocytosis machinery expression	12-48 hr
Metaplasticity Rules	Activity history	Altered LTP/LTD thresholds	Kinase/phosphatase expression changes	24-72 hr
Energy Allocation	Chronic demand	Mitochondrial positioning	Motor protein (kinesin, dynein) adaptation	12-48 hr
Network Integration	System-level activity	Presynaptic inhibition/facilitation	Interneuron connectivity changes	24-96 hr

MODULATION CASCADES (How Signals Propagate Through Events)

Example 1: High-Frequency Activity Cascade

Event Timeline:
t = 0 ms: First AP → Release (high Pr)
t = 10 ms: Second AP → Enhanced release (residual Ca²⁺ facilitation)
t = 100 ms: Tenth AP → Depressed release (RRP depletion, eCB inhibition)

Modulation Cascade:
Milliseconds: CDI develops, buffers saturate
Seconds: mGluR autoinhibition builds, ATP depletes
Minutes: BDNF synthesis begins, pumps adapt
Hours: VGCC expression increases, active zone grows
Days: Synapse strengthens, metaplasticity changes

Example 2: BDNF-Mediated Potentiation Cascade

Event Timeline:
t = 0-5 min: BDNF binds TrkB receptors
t = 5-30 min: Kinase cascades activate
t = 30-120 min: Local protein synthesis
t = 3-24 hr: Structural changes

Modulation Cascade:
Milliseconds: Immediate VGCC phosphorylation (↑ open probability)
Seconds: Enhanced vesicle priming (↑ RRP replenishment)
Minutes: More SNARE complexes synthesized (↑ release sites)
Hours: Additional VGCCs inserted (↑ Ca²⁺ influx capacity)
Days: Active zone expansion (↑ number of release sites)

Example 3: eCB-Mediated Depression Cascade

Event Timeline:
t = 100 ms: eCB produced postsynaptically
t = 200 ms: CB1 activation inhibits VGCCs
t = 1-10 min: β-arrestin recruitment, CB1 internalization
t = 1-24 hr: CB1 downregulation, structural changes

Modulation Cascade:
Milliseconds: Direct VGCC block (↓ Ca²⁺ influx)
Seconds: Reduced cAMP → ↓ PKA → ↓ vesicle priming
Minutes: CB1 internalization → reduced inhibition
Hours: CB1 downregulation → reduced modulatory capacity
Days: Synaptic downscaling → smaller active zone

CROSS-EVENT MODULATION PATTERNS

Feedforward Loops:

Positive:
AP → Ca²⁺ influx → CaMKII activation → ↑ VGCC phosphorylation → ↑ Ca²⁺ influx

Negative:
AP → Glutamate release → mGluR activation → ↓ VGCC opening → ↓ Ca²⁺ influx

Feedback Loops:

Homeostatic:
High activity → Ca²⁺ buildup → gene expression → more buffers/pumps → ↓ Ca²⁺ buildup

Trophic:
Activity → BDNF release → structural growth → more release sites → more activity

Energy Management Loops:

Energy Conservation:
ATP depletion → AMPK activation → ↓ protein synthesis → ↓ energy consumption

Energy Investment:
BDNF + adequate ATP → mTOR activation → protein synthesis → structural growth

INTEGRATED TIMELINE OF A RELEASE EVENT WITH MODULATIONS

t = 0-1 ms (AP to Release)

Primary events: AP → VGCC open → Ca²⁺ influx → Release
Modulations active:
  - AP shape from previous activity (K⁺ channel adaptation)
  - VGCDI from recent Ca²⁺ history
  - SNARE priming from recent ATP availability
  - Buffer capacity from expression levels

t = 1-100 ms (Immediate Aftermath)

Primary events: Ca²⁺ clearance, vesicle recycling initiation
Modulations active:
  - Residual Ca²⁺ facilitating next release
  - mGluR autoinhibition building
  - eCB production if postsynaptic Ca²⁺ high
  - ATP consumption for pumps and recycling

t = 100 ms - 1 min (Short-term Adaptation)

Primary events: RRP replenishment, metabolic adjustments
Modulations active:
  - Kinase/phosphatase balance shifting
  - NO production if sustained activity
  - BDNF release if activity pattern significant
  - Mitochondrial Ca²⁺ uptake affecting ATP

t = 1 min - 1 hr (Medium-term Plasticity)

Primary events: Protein synthesis, trafficking changes
Modulations active:
  - Local translation of VGCCs, SNAREs
  - Receptor trafficking to/from surface
  - Mitochondrial positioning changes
  - Metabolic pathway adjustments

t = 1 hr - days (Long-term Restructuring)

Primary events: Structural changes, gene expression
Modulations active:
  - Nuclear signaling for gene expression
  - Cytoskeletal reorganization
  - Synapse growth/shrinkage
  - Metaplasticity rule changes

MODELING PRINCIPLES

1. Hierarchical Modulation: Faster events constrain slower adaptations
2. Bidirectional Coupling: Every modulation affects and is affected by release events
3. Energy Constraints: ATP availability gates all processes
4. History Dependence: Past activity patterns determine current modulation states
5. Context Specificity: Same modulator can have opposite effects depending on timing/context
6. Redundancy: Multiple pathways can achieve similar outcomes
7. Trade-offs: Every enhancement has costs (energy, stability, flexibility)

This model shows how a simple millisecond release event sits at the center of a rich, multi-timescale regulatory network where each release both responds to and influences modulations across seconds to days, creating a dynamic, self-tuning system for synaptic transmission.